Gas analysis apparatus and method using parallel gas density detectors and different carrier gases to determine molecular weight

ABSTRACT

A method and apparatus are provided for measurement of a function of the molecular weight of a volatile chemical compound of a sample and the absolute weight contained therein by dividing a flow of volatilized sample into a plurality of portions having a fixed volume proportion to each other, trapping the chemical compound from each of said portions separately in trapping means, transferring the chemical compound of each of said portions into different carrier gases, said carrier gases differing in molecular weight from each other and from the chemical compound of the sample to form separate flows and passing the separate flows through separate means of the recording gas density cell type whereby signal outputs are obtained, the ratio of any two signal outputs being a function of the molecular weight of the compound, the means for trapping of the chemical compound to be measured preferably including a chromatographic tube when more than a single component is present.

United States Patent Paul 1 1 Sept. 26, 1972 [54] GAS ANALYSIS APPARATUSAND METHOD USING PARALLEL GAS DENSITY DETECTORS AND DIFFERENT CARRIERGASES TO DETERMINE MOLECULAR WEIGHT [72] lnventorz' Donald G. Paul,Kennett Square,

[73] Assignee: Chemalytics Corporation, Unionville, Pa.

[22] Filed: Nov. 9, 1970 [21] Appl. No.: 87,680

Parsons, Bracket Method for Molecular Weight Determination of PyrolysisProducts Using Gas Chromatography with a Gas Density Detector,Analytical Chemistry, Vol. 36, No. 9, August, 1964, pp. 1849- 1852.

Lovelock et al., The Palladium Transmodulator: A New Component for theGas Chromatograph, Analytical Chemistry, Volume 41, No. 8, July, 1969,pp. 1048- 1052.

Primary Examiner-Richard C. Queisser Assistant Examiner-C. E. Snee, lllAttorney-Mortenson and Weigel 57 ABSTRACT A method and apparatus areprovided for measurement of a function of the molecular weight of avolatile chemical compound of a sample and the absolute weight containedtherein by dividing a flow of volatilized sample into a plurality ofportions having a fixed volume proportion to each other, trapping thechemical compound from each of said portions separately in trappingmeans, transferring the chemical compound of each of said portions intodifferent carrier gases, said carrier gases differing in molecularweight from each other and from the chemical compound of the sample toform separate flows and passing the separate flows through separatemeans of the recording gas density cell type whereby signal outputs areobtained, the ratio of any two signal outputs being a function of themolecular weight of the compound, the means for trapping of the chemicalcompound to be measured preferably including a chromatographic tube whenmore than a single component is present.

13 Claims, 6 Drawing Figures PATENTED SW25 I973 3,693,403

so v 7 52 4a 68 12 0 46 as SFe SP6 N2 N2 FIG. I.

INVENTOR DONALD G. PAUL 8ZMM 'EM7 ATTORNEYS BACKGROUND OF THE INVENTIONThis invention relates to a method and apparatus for analysis ofvolatile substances as a function of molecular weight and moreparticularly to a method and apparatus for direct determination ofmolecular weight of substances.

It is known to determine molecular weight by the use of a gas densitydetector cell (gas density balance) in which a ratio of electricalresistance between upper and lower electrical elements varies by heatinterchange created from difference of flows of sample, carrier gas andreference gas as measured with a bridge circuit.

This prior art procedure involves programming a flow from a gaschromatography unit to a gas density balance and measuring theelectrical resistance as it changes during flow of the sample throughthe gas density cell. The gas density balance divides a flow of areference gas into an upper and a lower path the rate of flow of eachbeing measured by electrical detectors. When a sample in the samereference gas as a carrier gas is fed into the cell, a sample ofmolecular weight (gas density) lower than that of the reference gasrises to the upper path and retards the flow of the upper path and adifferential between the electrical output of the detectors is created.Conversely, a sample of molecular weight higher than that of thereference gas falls to the lower path and retards the flow of the lowerpath so that a differential which is electrically measurable by theoutput of the detectors is similarly produced. By repeating the processwith a second carrier gas different from the first used gas and usingidentical flow rates in the same apparatus Liberti et al. (atti. accad.nazl. Lincei. Rend. 20,623( 1956) obtained molecular weight resultsaccurate to within 4 percent.

The procedure of Liberti et al. required exact replications of sampleweights, identical replication of flow rates, heating temperatures,sample treatment, and recording rate. Moreover, any handling and losserrors were required to be identical. For these reasons, no success inpractical use of the procedure for molecular weight determination ofunknown samples ever evolved. In spite of the rapid growth in gaschromatography, developments since the discovery of Liberti et al. havealways taken paths which required more complicated additional apparatusfor qualitative analysis of the sample, such as mass spectrometry,infra-red analysis and the like. Since the retention time of a sample inthe chromatographic system does not necessarily deliver components of asample in order of molecular weight these later developments have notled to a simple method of determination of molecular weight withoutresort to further analysis.

Now it is an object of the present invention to provide a method andapparatus for the direct determination of the molecular weight of achemical compound.

It is a further object to provide a method and apparatus for determiningthe molecular weights of individual components of a chemical mixture.

It is a still further object to provide a method and apparatus forsimultaneous determination of retention times and molecular weights ofchemical compounds for qualitative analysis thereof.

It is also an object to provide a method and apparatus for determinationof both molecular weight and ab- 5 solute weight of individualcomponents of a chemical mixture.

lt is a still further object to provide a method and apparatus fordirect determination of molecular weight and/or absolute weight of achemical compound in a mixture using a single sample and two differentcarrier gases.

It is yet another object to provide a method and apparatus fordetermination of the molecular weight of a I a portion of said sample infixed proportion to the whole; a plurality of trapping means fortrapping the chemical compound from each flow and subsequently releasingthe compound into a different carrier gas in each trapping means; meansfor introducing a different carrier gas into each trapping means to forma flow of said compound in the respective different carrier gas; meansincluding a plurality of gas density detectors flowably connected toeach trapping means for detection of the chemical compound containedtherein and flowably connected to a flow of reference gas of the samechemical composition as the carrier gas containing the sample portion,said gas density detector being capable of providing an electricalsignal that varies in accordance with the flow rate of the reference gasand the flow rate of the reference gas as modified by the compoundflowing from said trapping means into the detector cell.

These objects are further achieved by the method of dividing a samplecontaining a chemical compound into a plurality of portions having afixed quantitative proportion to each other, trapping the chemicalcompound to be measured from each portion, transferring the compound soseparated from each portion into a different carrier gas to form aplurality of flows of the compound in different carrier gases, passingeach said flows into a separate signal recording gas density detector inwhich each gas density detector cell is provided with reference gas ofthe same composition as the carrier gas and recording the signal outputof the detector cells during the period of flow, the ratio of two signaloutputs being a function of the molecular weight of the compound.

BRIEF SUMMARY OF THE INVENTION In contrast with the prior art using thegas density detector for qualitative analysis of a volatile chemicalcompound in gas chromatography apparatus using repeat analyses in twodifferent carrier gases, the present invention provides a method andapparatus for direct determination of both retention time and molecularweight whereby the chemical compounds are analyzed both qualitativelyand quantitatively.

In the process of the present invention, the sample containing achemical compound is divided into a plurality of portions having a fixedquantitative proportion to each other. The chemical compound in eachportion is then individually trapped and subsequently transferredseparately into different carrier gases so as to provide a pluralityofflows of sample in the different carrier gases. Each flow of thechemical compound in its respective different carrier gas is then passedinto a separate signal recording gas density detector of the gas densitycell type in which each gas density cell is provided with a flow ofreference gas of the same composition as the carrier gas in the sampleflow. The signal output of the detector cells is recorded during theperiod of flow so as to form at least two curves, the ratio of the areasunder which is a function of the molecular weight of the chemicalcompound.

The sample is usually divided into two portions which are treatedsimilarly in two different carrier gases, but there are advantages inanalysis using division into three or four portions and treatingsimilarly in different carrier gases.

The carrier gases are substances differing in composition and molecularweight from the compounds in the sample to be analyzed. The carriergases used are usually single component compounds which pass through theapparatus without change in composition. One carrier gas preferably isof higher molecular weight than the compound of the sample to beanalyzed and the other is preferably of lower molecular weight than thecompound of the sample to be analyzed. The carrier gases which areuseful are chemical elements or compounds having molecular weights from2 to 200 and even higher by using suitably high temperatures in theapparatus. Examples are: hydrogen, helium, argon, neon, krypton,nitrogen, carbon dioxide, carbon tetrafiuoride, chlorotrifluoromethane,bromotrifluoromethane, dichlorodifluoromethane, trichlorofluoroethane,perfluoropropane (C F sulfurhexafluoride (SF octafiuorocyclobutane and1,1- difluoroethane. Hydrogen is rarely used because of its reactivityand possible explosion hazard. It is usually desirable to use compoundsof purity better than 99 percent. Helium, nitrogen, or carbon dioxide isquite satisfactory for use as the low molecular weight carrier gas andsulfur hexafluoride or octafluorocyclobutane as the high molecularweight carrier gas.

The chemical compound of each sample flow is trapped in a trapping meansdescribed hereinafter. When the sample to be analyzed contains a singlecompound, the trap is a simple means for condensing or adsorbing thecompound on the surface area of the trap. The atmosphere of the trap isthen replaced by carrier gas and the sample is transferred into thecarrier gas by heating the trap while flowing carrier gas through thetrap to produce a flow of the compound in the carrier gas.

When the sample contains more than a single compound the trapping meansis a two-stage trapping means and the mixture of compounds which arecomponents of the sample is first trapped by a simple trap in the firststage and the mixture is transferred into the carrier gas by heating thetrap while flowing carrier gas through the first stage trap to produce aflow of the mixture in carrier gas. The flow of mixture in the carriergas is then passed into and through a chromatographic tube where thereis a difference in retention times for the individual chemical compoundcomponents and, as a result, effluent from the chromatographic tube is aseries of flows. Each flow of the series contains a single chemicalcompound in the carrier gas, the flows being in the relative order oftheir respective retention times.

Generally, the chromatographic tubes for the plurality of flows aresubstantially identical as to the retention properties. However, thereare advantages in having them different so as to have differentretention times. The retention times and molecular weights takentogether are useful in identification as to chemical structure.

BRIEF DESCRIPTION OF THE DRAWINGS The apparatus of the invention isbriefly described for simplicity showing apparatus for dividing a sampleinto a pair of flows in two different carrier gases. It is to beunderstood that apparatus for dividing the sample into a plurality offlows in which three different carrier gases are used requires a dividerfor splitting the sample into three flows and three trapping means andthree detectors similar to the diagrams wherein only two flows areshown.

FIG. 1 is a diagram of a complete apparatus provided in accordance withthe invention with a showing of the routing of flows in a first step ofprocedure.

FIG. 2 is a view similar to FIG. 1 but showing a second step ofprocedure with omission of portions of the apparatus irrelevant to thatstep;

FIG. 3 is a view similar to FIG. 1 but showing a third step of procedurewith irrelevant portions of the apparatus omitted;

FIG. 4 is a view similar to FIG. 1 but showing a fourth step ofprocedure with irrelevant portions of the apparatus omitted;

FIG. 5 is a diagram of a pair of records produced by recorders and whichare to be compared;

FIG. 6 consists of the more important equations explanatory of thetheory and practice of operation.

DETAILED DESCRIPTION FIG. 1 shows the apparatus as a whole, indicatingthe routing of flow in the first step of operation. Where valves areindicated, it is understood that many of these are used in gangedcombination so that the control of one simultaneously controls anotherin the gang.

Describing first a particular construction, an inlet 2 may serve for theintroduction, from a source under pressure, of a carrying gas which maybe of any suitable type into which there is subsequently introduced thesample for which analysis is desired. This connection runs to a splittertee 4 to provide division of flow through fiow restrictors 7 and 9, intoconduits 6 and 8, respectively, which restrictors may be physicallyprovided by capillary tubing which need not be exactly matched in sizeand length. The reason for these restrictors, functioning like highelectrical resistors, is to provide a reproducible division of two highspeed flows essentially independent of variations in conditions beyondthe conduits 6 and 8 which might affect the division of flow. Forexample, as will presently appear, the flows run to traps, changes oftemperature of which, or changes of other conditions of which, canchange the ratio of the respective flows, whereas if sufficientresistances are interposed by restrictors 7 and 9 the disturbancesdownstream will have negligible effect. ln addition, the resistancesessentially eliminate any gravitational effects, the resistors givingrise to high linear rates of flow proportional to pressures at the tee 4due primarily to accelerations. The respective flows need not be equal;in fact, in many instances inequality will produce better results inview of different conditions in later parts of the apparatus and theiroperations. What is required is a nearly fixed ratio of the two flows sothat results obtained in the handling of unknowns will be comparablewith the handling of known materials during calibration.

The gas introduced at connection 2 may be provided from a supply bottleunder pressure with regulation of rate of flow through a flow regulator10 which, like other regulators hereafter described may be of thewellknown type commonly used in gas chromatography. Beyond the flowregulator, there is a sample port 12 for introduction of solid, liquidor gaseous sample in conventional fashion. This injection port ispreferably suitably heated so that the injected sample will be vaporizedso as to be carried by the gas. It is desirable that immediatelyfollowing the injection port there should be an enlarged chamber 13 toprevent an undue rise of pressure caused by sudden vaporization of thesample, better uniformity of the division being obtained when thepressure at the tee 4 remains approximately constant. Such a chamber isparticularly desirable since the conventional regulator present at It)usually has a rather slow response.

The dividers, capillary tubing, chromatograph tubes, gas density cells,valves and other parts of the apparatus may be made of glass, metal suchas stainless steel, plastic or other material selected to be suitablefor the particular samples and carrier gases used.

The foregoing describes a particular arrangement and operation in whichthe carrying gas is at an elevated pressure and the sample is injectedbeyond a regulator. However, the sample may be present in the carryinggas as it enters at 12, for example, if there is being analyzed a samplegas containing impurities, e.g. polluted air. While for consistency ofdescription there will be referred to entry of gases under pressure, itwill be evident that all that is required is a pressure gradient orgradients to produce flow, and consequently flow may be provided by theapplication of suction to various points which are referred tohereinafter as vents, with input at any desired pressure, for exampleatmospheric if contents of air are to be analyzed.

While the use of gas to carry the sample through the restrictive dividercapillary tubes 7 and 9 is convenient and practical, it is to beunderstood that there are also advantages to the application of asuction in the flow lines to be described hereinafter for drawing thevaporized sample into the system through these capillary tubes.

The carrying gas for conveying the sample through the first part of thesystem up to the step of trapping the two portions of sample in theirrespective traps is not normally critical as to purity, since it isdisplaced subsequently by substantially pure carrier gases used fortransferring the two portions of sample from their respective traps tothe chromatographic separators. The substantially pure carrier gasesused in this second step are selected from the gases known in the artand particularly those listed in the description of the method asdescribed above.

The apparatus includes as components various valves which are manuallyset to selected positions, and those valves are most effectively shownby way of disclosure of their ports in the drawings, with indication insolid lines of connections which exist during the several steps in theoperation, while in FIG. 1 alternative connections, not used in theinitial step, are indicated in dotted lines. The physical valvesthemselves are not detailed since they are of conventional types andreadily available.

The conduit 6 runs to valve 14 which, in the initial step, provides flowto valve 16 and thence to valves 18 and 20 and from valve 20 intotrapping column 22. The valves are selectively operated or ganged wherethis is convenient and possible, so as to control the flow of gases asdesired.

The apparatus is essentially symmetrical, and flow takes place fromconduit 8 through the valves 24, 26, 28 and 36) into trapping column 32.

The right hand side of FIG. 1 shows valving which includes valves 34 and36 and the left hand shows valving which includes valves 38 and 40 whichare not utilized in the initial step of operation. The valves 34 and 38are connected to respective vents 42 and 44.

At 46 there is an inlet connection which, for consistency of descriptionmay be said to receive a carrier gas such as nitrogen under pressure,the flow of which gas is controlled by a conventional regulator 48, sothat the gas under controlled flow passes to valve 36 in a subsequentstage of operation. In similar fashion an inlet 50 is provided andconnected to a pressure source of another gas which for consistency ofdescription may be considered to be sulfurhexafluoride (SF the flow ofwhich is regulated by a regulator 52 from which a conduit runs to thevalve 40. The gases nitrogen and sulfurhexafluoride serve in manyinstances as suitable carriers, the nitrogen having a molecular weightwhich is low and generally less than the molecular weights of thecomponents of the samples, while the sulfurhexafluoride has a highmolecular weight which is often greater than the molecular weights ofthe components of the samples and is of quite general utility because ofits inertness and its high molecular weight.

The lower end of trapping column 22 is connected to a valve 54 tocontrol flow either to a valve 56 which is connected to a vent, or toprovide flow to another valve 58.

On the other side of the apparatus the lower end of the trapping column32 is connected to a valve 60 which provides connection to a ventingvalve 62 or alternatively to a valve 64.

When the sample is drawn through the divider capillaries 7 and 9 bysuction as described above, venting valves 56 and 62 are attached to asuction means such as a vacuum line so as to draw sample portions intothe traps.

An inlet 66 connected to a source of nitrogen under pressure provides aflow of nitrogen through a flow regulator 68 and an auxiliary injectionport 70 to the valve 58. in similar fashion an inlet 72 connected to asource of sulfurhexafluoride under pressure provides a flow ofsulfurhexafluoride through a flow regulator 74 and thence through anauxiliary injection port 75 to the valve 64.

The auxiliary sample injection ports 70 and 75 are provided for theinjection of known samples for calibration purposes as well as forquantitative analysis.

These ports are used, for example, when samples are analyzedquantitatively for components of known molecular weights. In suchqualitative analysis it is necessary to use only one side of theequipment and only one carrier gas. In this case the carrier gas whichgives the largest response is used. Thus, it is advantageous to use thehigh molecular weight carrier gas such as sulfurhexafluoride inanalyzing low molecular weight components and the low molecular weightcarrier gas such as nitrogen or helium in analyzing high molecularweight components.

A valve 76 is connected to the lower end of a gas chromatograph column78, while a corresponding valve 80 on the other side of the apparatus isconnected to the lower end of a gas chromatograph column 82.

At this point it may be remarked that while, for simplicity ofdisclosure, multiple inlet connections to pressure sources of nitrogenand sulfurhexafluoride are indicated, and will be further indicated,these connections may be made to the same supplies of the respectivegases and the several flow regulators may in some instances be the sameregulators when they are not required to operate simultaneously, therouting of the gases being provided by suitable valving as will beobvious. Close-off valves, not known, are provided to open and shutinlets in obvious fashion at the sources.

Symmetrically provided in the apparatus are detectors such as densitymeters 84 and 86 which are of a well-known type described in detail inUS. Pat. No. 3,050,984, dated Aug. 28, 1962, and elsewhere.

Each of the detectors 84 and 86 is of the type in which verticalpassages are provided which have reception zones at their respective midportions for reception of reference gas and for reception of the samegas carrying a sample. The reference gas passes in heat conductiverelation with a pair of thermally responsive electrical detectorsconnected in a bridge circuit providing an output to a recorder which ispart of each detector (though a dual recorder may serve for both) andnot known per se. The gas containing the sample controls the flows ofthe reference gas by becoming admixed with the flowing reference gas,and the combined gases pass to a vertically centered outlet. Theconduits connected to other apparatus are as follows:

The conduit 88 runs from the upper end of the column 78 to the midportion of a vertical passage 89 in the detector 84, and in similarfashion a conduit 90 runs from the upper end of column 82 to the midportion of a vertical passage 91 in the detector 86. An inlet 92connected to a pressure source of nitrogen runs through flow regulator94 and then through conduit 96 to the mid portion of the verticalpassage 97 at the upper and lower ends of which connections are made tohorizontally running extensions of the upper and lower ends of thepassage 89. The flows unite and the combined discharge passes throughthe outlet conduit 99. This outlet may either be a vent, or it may be aconduit leading to a trapping column in the event the sample is to beretained for some further operation.

In similar fashion an inlet 98 connected to a pressure source ofsulfurhexafluoride runs to a flow regulator 100 and thence throughconduit 102 to the mid portion of the vertical passage 103, whichcorresponds to 97. The flow passages are similar to those previouslydescribed, outlet flow taking place through the conduit 105.

Indicated in the detector 84 are the respective upper and lower elementssuch as thermally-responsive elements 104 and 106, such as filaments orthermistors, while corresponding elements are designated 108 and 110 inthe detector 86. The operation of the detectors is a conventional one inwhich variations in the upward and downward flows of the reference gasesproduce electrical unbalance of the thermally-responsive elements so asto provide operation of the corresponding recorder.

The various trapping columns 22 and 32, chromatographic columns 78 and82, and detectors 84 and 86 are provided with heaters which areindicated in FIG. 1 at I-I to H, inclusive. These heaters for theabsorption columns and the density detectors are arranged to provideproper temperatures, thermostatically controlled, at various stages ofthe operation not only to these major components but also to connectingconduits with the objective of maintaining thermal stability, and in thecase of the absorbing columns, also to provide conditions for holdingand vaporization of the sample material. The temperatures maintainedare, of course, selected to suit the particular materials which areinvolved.

For the same purpose and also to maintain gas compositions constant,except when intentionally changed, flows of carrier gases are maintainedin all parts of the system whether actively involved in carrying out theindividual steps of operation or not.

Reference may now be made to the operational steps in making aquantitative analysis of the constituents including their identificationby their molecular weights in apparatus which has been calibrated and isready for use. The operations will be first described without referenceto theory.

FIG. 1 indicates in full lines the connections which are involved in theinitial part of the operation which effects a division of a sample to beanalyzed into two definitely related portions. In this initial phase ofoperation the trap heaters are ordinarily off (or at low heat) with thetrapping columns 22 and 32, particularly at room or other lowtemperature, although if highly volatile materials are contained in thesample these may even be subjected to refrigeration. Assuming, for

generality, that the carrier gas in the introduction step,

is a gas other than nitrogen or sulfurhexafluoride, it is desirable, inorder to secure definiteness of division of the sample, when using acarrier gas before division, to have the carrier gas flow for a periodpreceding the sample introduction so as to secure uniformity of gascontent particularly in the columns 22 and 32. Following this, anoperable quantity of the sample is introduced into the injection port 12while the gas is flowing from the inlet 2 and while heating theinjection port 12, if necessary to maintain volatility, so as toaccelerate vaporization of the sample. Definite division of the samplestakes place at the splitter tee 4 from which the split flows are led tothe columns 22 and 32.

The columns 22 and 32 are high surface chambers and usually contain anadsorbing material such as any of a great variety of adsorbing materialswell-known in the art of gas chromatography, for example: Alumina,silica, clay, metal silicates, acrylic polymer beads, Apiezon L,Carbowax M, Silicone OV- 7, Squalane, Poropak P, Q, R, S, or T, glassbeads, Teflon, activated charcoal, or mixtures. Various of those listedare proprietary products, sold under trademarks. The actions involvedare essentially physical, and in the adsorption the action is in manycases essentially condensation, either on more or less inert porousmaterial or in viscous liquid or semi-liquid coatings on inert carriers,or true molecular adsorption. The action involved in removal of theconstituents is the reverse, i.e., essentially vaporization. Chemicalreaction is not usually involved; and as will be pointed out later, theorder of desorption that occurs is not entirely a-function of molecularweight, or of chemical composition.

The adsorbing materials such as those just indicated have the property,when cold, of very quickly adsorbing the sample constituents from theinfluent gas with the desirable result that for all intents and purposedthe entire content of the sample is held at the upper ends of thesecolumns. It is not desirable at this stage to have the adsorption takeplace in a chromatographic fashion, i.e., distributed along the column,but rather the adsorption is desirably in small slugs occupying littleof the column length with the ultimate objective of providing a compactsource from which the sample materials in the two columns may bepractically simultaneously and in a very short period driven off withheat and delivered into the chromatograph columns 78 and 82, thusincreasing column efficiency.

At the time this sample-depositing step is carried out,

it is convenient to prepare other portions of the apparatus for use,although this is merely a cleaning and stabilizing operation, by causingnitrogen to enter at inlet 66 and to pass into the bottom of column 78and from it through conduit 88 to sweep through the passages in thedetector 84 which also receives nitrogen from the inlet 92. A similarcleansing and stabilizing operation is carried out through thecorresponding elements in the left hand portion of the apparatus byintroducing sulfurhexafluoride at the inlets 72 and 98.

The second step in the operation is illustrated in FIG. 2 which, forsimplicity, omits those elements of the apparatus which are not involvedin this step, the auxiliary injection ports 70 and 75 being alsoomitted, though flow takes place therethrough, and also through theother passages to maintain stabilization.

The purpose of the second step is to replace in the columns 22 and 32the carrying gas which was used to introduce the samples by,respectively, nitrogen and sulfur hexafluoride. One of these gases issuitably al ready in its proper half of the system if the same gas hadbeen used originally by introduction at inlet 2. in the right handportion of the apparatus, nitrogen is introduced at 46 and flows throughthe conduits illustrated downwardly through the column 22 (which remainscold) with venting at 56. In similar fashion sulfurhexafluorideintroduced at 50 passes downwardly through the column 32.Theseoperations are carried out sufficiently to insure that the twoportions of the apparatus contain as gases the nitrogen andsulfurhexafluoride respectively. The conduit and valve connections tocolumns 78 and 82 and to the detectors 84 and 86 are the same asillustrated and described with reference to FIG. 1.

While the second step just described is desirable, and in some casesnecessary, depending upon the carrier gases and the composition to beanalyzed, in other instances this step may be omitted and when this isthe case there may be corresponding simplification of the apparatusparticularly in the matter of the valve connections preceding thetrapping columns. Hence the described second step is not of the essenceof the procedure.

The nest step is illustrated in FIG. 3, from which irrelevant portionsof the apparatus are omitted, (though flows may continue for stabilizingas mentioned) and involves the purging of the samples from the trappingcolumns 22 and 32 into the chromatograph columns 78 and 82. The formerare now heated and the latter remain cool, or are adjusted to a lowertemperature than the trapping columns as necessary to get optimumabsorption conditions. Nitrogen introduced at 66 is routed through theregulator into the lower end of column 22 and from which it removes theslug of sample and introduces it into the chromatograph column 78. Inthis last column, the stratification occurs in which the more volatilecomponents of the sample usually become located higher in the columnthan the less volatile components. Nitrogen freed from the componentsnow located in the column 78, flows outwardly and may be convenientlyrouted as indicated through the passages in the detector 84. Similaroperations, effected by sulfurhexafluoride entering at 72 take place,displacing the slug of sample from column 32 into the chromatographcolumn 82, the former being heated and the latter remaining cool orbeing cooled or heated at a lower temperature.

The columns 78 and 82 suitably contain chromatographic absorbingmaterial of any of a wide variety of types, and these materials may bethe same as those previously described as used in the trapping column.From the many column fillers which are available choices may be made ofthose giving best results for particular classes of materials andcarriers, though little differences are noted in comparing theoperations of different adsorbing materials. Controlled temperatures areselected to secure optimum results. A trapping" action may be producedat lower temperatures whereas at higher temperatures betterstratification may occur, with separation of the components rather thantheir accumulation in limited slugs. The separation effective in thesecolumns provides for the later introduction into the detectors of thesample components in a sequence beginning with the most volatile andending with the least volatile.

The final step'of operation is illustrated in FIG. 4, there being hereomitted, as before, the irrelevant portions of the apparatus throughwhich flows continue, as stated. What is carried out in this final stepis the displacement of the sample components, beginning with the mostvolatile into and through the detectors to pro vide records; Nitrogenintroduced at 66 has its rate of flow'controlled by regulator 68 andenters the column 78 to displace the sample components through thedetector 84. The column 78 is heated to a temperature suitable for thecomponents involved and the ambient temperature of the detector 84 isalso maintained constant and also at a suitable value. The detector nowoperates in conventional fashion, reference nitrogen being introduced ata regulated flow rate through the connection 92 and the flow regulator94. As is known for this type of detector, the relative densities of thetwoflows now come into play so that the rates of flow of the referencenitrogen past the elements 104 and 106 create unbalance of theelectrical bridge which is recorded. The same type of operation occursin the left hand portion of the apparatus, except thatsulfurhexafluoride now constitutes the displacing gas and also thereference gas. There is, of course, a reversal of the conditionsinvolved in that the reference nitrogen would have a lower density thannitrogen containing the components heavier than it, while the referencesulfurhexafluoride would have a greater density than the portions ofsulfurhexafluoride containing the sample components lighter than it.This fourth step of programmed elution to detectors using the nitrogenand sulfurhexafluoride carrier gases may be carried out eithersimultaneously or successively, the same electrical recorder beingpossibly used in the latter instance.

When a single component sample is being analyzed, it is possible to useonly the first named traps by making the exchange to pure carrier gastherein and then heating the traps so as to form an admixture with thecarrier gas so as to create flows which carry the sample in pure carriergas directly to the gas density cells. The sample in the trap is allowedto flow from either its inlet end or from its outlet end directly to thegas density cells. This procedure differs from the base in which the gasis passed to chromatographic tubes for separation of a multicomponentsample, in that a multicomponent sample is preferably allowed to passfrom the first named trap through the inlet end by reversal of gas flow.The reason for removal from the trap by reversal of flow is so that noseparation of sample takes place prior to the development of separationin the chromatographic tubes.

The advantages of the apparatus and procedure of the present inventionwill now be clear. The sample is divided into two portions being a knownquantitative relationship to each other and are handled in substantiallyidentical fashions. Any difference in handling, due to the use of thetwo carrying gases, is constant from run to run, and from the standpointof the production of the density records the result achieved in eachrecorder is the same resulting in direct comparison so that in the tworecords the components can be easily identified. As is well know, thequantitative amount (weight) of a constituent is determined by the areaof the peak produced in the recorder chart as compared to an areaproduced by a standard sample. With proper rates of flow through thechromatograph columns, the records consist of a constant baseline graph(resulting when the components are not passing through the detector)with peaks occurring whenever the components are passing. The sequenceof the peaks and their spacings can be recognized as definitive of thecomponents. This recognition, of course, is afforded particularly bycalibration. The calibration is carried out by the introduction of knownsamples at the auxiliary injection ports and 75, the nitrogen andsulfurhexafluoride being introduced at controlled rates first to buildup the array of the components in the chromatograph columns 78 and 82and then to transfer them serially through the respective detectors togive basic graphs corresponding thereto. Maintenance of constant chosenconditions makes the results comparable so as to achieve high accuracyof identification and measurements.

Under some conditions simplifying variations of the apparatus may beused, For example, it is possible that the trapping columns 22 and 32may be bypassed by introduction of the sample in the two flows directlyinto the chromatograph columns 78 and 82 through injection ports 70 and75. However, it is more generally desirable to utilize the trappingcolumns 22 and 32 to accumulate first slugs of the divided sample, thusto avoid such differences as might arise from the direct introductionwhich introduces added error not involved in the usual injection andsplitting procedure above described. The slug accumulation on the traps,without any substantial separation of constituents, permits the furtheroperations to be carried out simultaneously under simultaneouslyexisting conditions as to temperature, pressure and flow rates.

With proper precautions and added valving for gas purging andintroduction of another gas, a single detector may, of course, be used.

FIG. 5 shows two graphs G and G, which are the comparable graphsproduced by the recorders operated by the signal outputs of the gasdensity cells. Each consists of a number of peaks, and under normalconditions of simultaneously feeding the series of flows from thechromatographic columns to the detectors,

the sequences of the peaks on the two graphs produced A will correspond,i.e., peaks which are produced at corresponding positions in the twosequences such as P and P,, represent the measurements of the samecomponent of an unknown (or of a known substance in calibration). Therelative areas of these peaks are of consequence and are determined bythe use of a planimeter or graphically, or by electronic integrator.

The operation of a density measuring cell, arbitrarily referring to oneof the pair shown, produces data which are related in accordance withequation (1) of FIG. 6, in which:

W is the actual weight of a particular component of the unknown sampleproducing a single peak in passing through the cell;

A is the corresponding area of the single peak in the graph of theresponse of that cell to the passage of that component;

K is the cell constant;

M, is the molecular weight of the carrier gas used for carrying thatcomponent through the cell, and also used as a reference gas.

M is the molecular weight of the carrier gas used for carrying thatcomponent through the cell, and also used as a reference gas.

Likewise the corresponding relation of data for the second cell is givenin equation (2) of FIG. 6 in which:

W, is the actual weight of a particular component of the unknown sampleproducing a single peak;

A, is the corresponding area of the single peak in the graph of theresponse of that cell to the passage of that component;

K, is the cell element constant which is separately determined M I isthe molecular weight of that component which produces the peak, and

M is the molecular weight of the second carrier gas used in that cell.

Since the total weight of the component, W, is the sum of W, and W andW, and W, are related according to the splitting ratio a, a

and from this the expression (3) and (4) follow:

By substituting (l) in expression (3), and (2) in expression (4) thereis obtained equation (5) in which (k,)/(k,) (a,)/( a K.

K is thus a constant for the apparatus which is determined by passingany known pure sample through the analytical process. Normal heptane isa satisfactory pure material which can be used for this purpose. When Kis substituted for (kg/(K a )/(a,) in equation (5 the following equationis obtained.

By designating K (Ag/(A to be a constant Y,

and YM, YM,,, M M which by rearranging becomes YM M YM M or (Y -l) M,

YM which on dividing by (Y 1) becomes equation (7) After solving for Kusing a known pure compound as indicated above and using one gas, Y canbe similarly found by repeating the process with a pure compound weightof the sample is calculated. If there are several components in a sampleresulting in several peaks the equation can be set up in a computer sothat only the ratio AJA, for each component needs to be changed for eachmolecular weight determination of the several components.

It is to be noted that when calibrating the apparatus with a puresample, the pure sample should be one with a molecular weight greaterthan that of the carrier gas used so as to correct for the slightdifference which may occur between pairs of gas density cell thermallyresponsive resistors.

The apparatus of this invention provides a simultaneous measurement oftwo portions of a sample whereby errors due to fluctuations intemperatures, flow rates, and pressures cancel out, and the constantsdetermined for the apparatus and used in the calculations are fixed.Moreover, the apparatus using the splitter and plural flow systemprovides easy and accurate control of heating and cooling, as required,in such a manner that a true comparison of the two portions of splitsample is obtained. As a result, accurate measurement of molecularweight of sample components is readily achieved.

While it is possible to use apparatus according to the present inventioncomprising a sample splitter, a plurality of traps, including whendesired, a simple trap and chromatographic tube trap in series and thennot a plurality but a single gas detector cell to analyze first thesample in one trap in one carrier gas and then a sample in another trapin a different carrier gas and thus simplify the apparatus, suchapparatus has one of the serious disadvantages of the prior art, namely,the time required to thoroughly clean the first carrier gas from thedetectors system before using it for the second different carrier gas.The present invention using a plurality of gas detectors overcomes thisdisadvantage. The present invention does not, however, exclude thenon-simultaneous analysis of the flows from the trap systems whereby asingle recorder can be used first for recording peaks of one flow andthen recording peaks of a second flow. Such a system is useful whenusing for example three chromatograph flows and only one dual penrecorder.

It is obvious that the process of this invention is not limited to thedivision of a flow of gas into two samples portions and comparison ofthe sample in two different gases, since additional sample portions canbe compared in a third and even a fourth different gas for greateraccuracy. Furthermore, columns with different packing materials can beused and this is particularly advantageous when a third and a fourthportion in a third and fourth different additional gas is used. The datatherefrom will provide greater accuracy and precision in identifyingcomponents of complex mixtures. When quantitative analysis is desired,the apparatus is calibrated with a known weight of a known compound, andthe molecular weight and absolute weight of one component of the sampleinjected must be known. In the case of an unknown sample, a knownpercent of a known compound can be mixed with the sample to produce asample for this purpose.

The above description applies more particularly to qualitative analysis.The formulas given above are used 'in this calculation, e.g., M

---- II Alan In the first calculation, k is calculated using a knownweight of the known sample t of known molecular weight M and knownmolecular weight of carrier gas M to obtain the area A,. Then this k isused in the same formula where t M, and A, are respectively weight,molecular weight and area under the curve for the unknown in the sample.

What is claimed is:

1. Apparatus for measurement of a function of the molecular weight of avolatile component of a sample comprising: I

a. means for dividing a flow of sample containing a chemical compoundinto a'plurality of flows each containing a portion of said sample in afixed proportion to each other;

. separate trapping means disposed in each of said flows for separatingand releasing each of said chemical compound portions from itsrespective flow,

c. means associated with each of said trapping means for entraining therespective separated chemical compound portions in different flows eachof a different carrier gas;

. detector means including a plurality of gas density detector cellscapable of providing an electrical signal that varies in accordance withthe difference in flow rates of a reference gas and said reference gasmodified by a content of a compound of different molecular weight, saiddetector cells being provided with a reference gas inlet and a samplegas inlet, said sample gas inlet being flowably connected with itsrespective trapping means.

2. Apparatus of claim 1 in which the sample dividing means is a meansfor dividing the sample into a plurality of flows not exceeding four.

3. Apparatus of claim 2 in which the sample dividing means is a meansfor dividing the sample into a pair of flows.

4. Apparatus of claim 3 in which each trapping means is heat controlledand is a high surface area vessel having substantially no differentialretention properties.

5. Apparatus of claim 3 in which each trapping means is a two stagetrapping means wherein the first stage includes means for transferring aplurality of chemical compound components from each sample flow as amixture into a carrier gas and the second stage includes means forseparating chromatographically said plurality of chemical compoundcomponents into a series of flows of individual chemical compounds inthe respective carrier gas.

6. Apparatus of claim 5 in which the two stage trapping means includes afirst stage trapping means having substantially no differentialretention properties for entraining the respective separated chemicalportions and a second, stage trapping means having substantialdifferential retention properties and valving means therebetween wherebyflow from the first stage trapping means to the second stage trappingmeans is controlled.

7. A method for measurement of the molecular weight of a volatilechemical compound component of a sample comprising: chemical compoundinto a plurality of flows of portions having a fixed quantitative ratioto each other;

a. simultaneously dividing a sample containing a chemical compound intoa plurality of flows of I portions having a fixed quantitative ratio toeach other;

b. simultaneously trapping the chemical compound from each of saidportions and simultaneously transferring said chemical compound of eachof said portions into a different carrier gas to form a plurality ofsample flows in respective different carrier gases;

c. simultaneously passing each said flow into a separate signalrecording gas density detector of the gas density. cell type in whicheach gas density cell is provided with a flow reference gas of the samecomposition as the carrier gas in the sample flow; and

. simultaneously recording the signal outputs of the detector cellsduring the period of flow, said signal outputs being a function of themolecular weight of said chemical compound.

8. The method of claim 7 in which the sample consists of an individualchemical compound in a carrier gas and steps b and c comprise the stepsof trapping each chemical compound, purging the carrier gas therefromwith said different carrier gas and detrapping each chemical compoundinto its respective different carrier gas.

9. The method of claim 7 in which the plurality of sample portions doesnot exceed four.

10. The method of claim 9 in which the sample is transferred into afirst carrier gas of different chemical structure than the chemicalcompound component of the sample to be measured to form a sample flowand the sample flow is divided into a plurality of sample flows, fromwhich flows each compound component is subsequently separated fromthefirst carrier gas before different carrier gases, each flowcomprising a flow of g the chemical compound component in the respectivecarrier gas serially separated from each other in order of thechromatographic retention time of each chemical compound.

12. The method of claim 11 in which the chemical components of themixtures in difierent carrier gases are chromatographically separated onchromotographic columns having different adsorbent properties so as toprovide a plurality of flows in which the retention times are differentin each flow.

13. The method of claim 12 in which both retention time and molecularweight are determined from the data as a means of chemicalidentification.

2. Apparatus of claim 1 in which the sample dividing means is a meansfor dividing the sample into a plurality of flows not exceeding four. 3.Apparatus of claim 2 in which the sample dividing means is a means fordividing the sample into a pair of flows.
 4. Apparatus of claim 3 inwhich each trapping means is heat controlled and is a high surface areavessel having substantially no differential retention properties. 5.Apparatus of claim 3 in which each trapping means is a two stagetrapping means wherein the first stage includes means for transferring aplurality of chemical compound components from each sample flow as amixture into a carrier gas and the second stage includes means forseparating chromatographically said plurality of chemical compoundcomponents into a series of flows of individual chemical compounds inthe respective carrier gas.
 6. Apparatus of claim 5 in which the twostage trapping means includes a first stage trapping means havingsubstantially no differential retention properties for entraining therespective separated chemical portions and a second stage trapping meanshaving substantial differential retention properties and valving meanstherebetween whereby flow from the first stage trapping means to thesecond stage trapping means is controlled.
 7. A method for measurementof the molecular weight of a volatile chemical compound component of asample comprising: chemical compound into a plurality of flows ofportions having a fixed quantitative ratio to each other; a.simultaneously dividing a sample containing a chemical compound into aplurality of flows of portions having a fixed quantitative ratio to eachother; b. simultaneously trapping the chemical compound from each ofsaid portions and simultaneously transferring said chemical compound ofeach of said portions into a different carrier gas to form a pluralityof sample flows in respective different carrier gases; c. simultaneouslypassing each said flow into a separate signal recording gas densitydetector of the gas density cell type in which each gas density cell isprovided with a flow reference gas of the same composition as thecarrier gas in the sample flow; and d. simultaneously recording thesignal outputs of the detector cells during the period of flow, saidsignal outputs being a function of the molecular weight of said chemicalcompound.
 8. The method of claim 7 in which the saMple consists of anindividual chemical compound in a carrier gas and steps b and c comprisethe steps of trapping each chemical compound, purging the carrier gastherefrom with said different carrier gas and detrapping each chemicalcompound into its respective different carrier gas.
 9. The method ofclaim 7 in which the plurality of sample portions does not exceed four.10. The method of claim 9 in which the sample is transferred into afirst carrier gas of different chemical structure than the chemicalcompound component of the sample to be measured to form a sample flowand the sample flow is divided into a plurality of sample flows, fromwhich flows each compound component is subsequently separated from thefirst carrier gas before being transferred into said different carriergas.
 11. The method of claim 10 in which the sample contains a pluralityof chemical components to be measured and the plurality of chemicalcomponents is first trapped from the carrier gas as a plurality ofmixtures of chemical components, each said mixture is transferred into adifferent carrier gas and chromatographically separated into a pluralityof flows in respectively different carrier gases, each flow comprising aflow of the chemical compound component in the respective carrier gasserially separated from each other in order of the chromatographicretention time of each chemical compound.
 12. The method of claim 11 inwhich the chemical components of the mixtures in different carrier gasesare chromatographically separated on chromotographic columns havingdifferent adsorbent properties so as to provide a plurality of flows inwhich the retention times are different in each flow.
 13. The method ofclaim 12 in which both retention time and molecular weight aredetermined from the data as a means of chemical identification.